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Unit 6

Electric Motors

Vocabulary and Pronunciation
Exercise 1 Match English terms and Vietnamese translations
English

Vietnamese

1. mechanical energy

Cổ góp

2. armature

Cơ năng

3. reverse polarity

cuộn dây kích thích

4. impedance

đảo cực

5. pulsating force

đầu máy xe lửa

6. traction motor

Động cơ kéo

7. electric railways

Động cơ xén cỏ

8. locomotives

Đường xe điện

9. torque

lực mạch động

10. compact design

Mạch tyristo

11. commutator

mô men xoắn, mô men quay

12. thyristor circuit

nửa chu kỳ

13. half-wave

Phần ứng

14. revolutions per minute (rpm)

Ray tiếp xúc

15. weed trimmer motors

số vòng quay trong một giây

16. field windings

thiết kế nhỏ gọn

17. third rail

Trở kháng

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Exercise 2 T.S 1

Listen and practise

1. mechanical energy

7. electric railways

13. half-wave

2. armature

8. locomotives

14. revolutions per
minute (rpm)

3. reverse polarity

9. torque

15. weed trimmer motors

4. impedance

10. compact design

16. field windings

5. pulsating force

11. commutator

17. third rail

6. traction motor

12. thyristor circuit

Exercise 3 T.S 2 Listen and complete the sentences by selecting the correct
word from the box to fill in the gap
mechanical energy

electric railways

half-wave

armature

locomotives

revolutions per minute (rpm)

reverse polarity

torque

weed trimmer motors

impedance

compact design

field windings

pulsating force

commutator

third rail

traction motor

thyristor circuit

1. An inside rotor attached to the output shaft that
is given a ……………………by the rotating
field.
2. An electric motor uses electrical energy to
produce …………………………
3. Universal motors can rotate at relatively
…………………………..(rpm).
4. a small fan blade attached to the
……………………acts as an artificial load to
limit the motor speed to a safe value.
5. By changing the …………………………connected to the rotor circuit,
the speed/current and speed/torque curves can be altered.
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reading:

Universal motors

Exercise 4 Read the passage carefully and decide whether the statements
are true (T) or false (F)
A variant of the wound field DC motor is the universal motor. The name derives
from the fact that it may use AC or DC supply current, although in practice they
are nearly always used with AC supplies. The principle is that in a wound field
DC motor the current in both the field and the armature (and hence the resultant
magnetic fields) will alternate (reverse polarity) at the same time, and hence the
mechanical force generated is always in the same direction. In practice, the
motor must be specially designed to cope with the AC current (impedance must
be taken into account, as must the pulsating force), and the resultant motor is
generally less efficient than an equivalent pure DC motor. Operating at normal
power line frequencies, the maximum output of universal motors is limited and
motors exceeding one kilowatt are rare. But universal motors also form the basis
of the traditional railway traction motor in electric railways. In this application,
to keep their electrical efficiency high, they were operated from very low
frequency AC supplies, with 25 Hz and 16 2/3 hertz operation being common.
Because they are universal motors, locomotives using this design were also
commonly capable of operating from a third rail powered by DC.
The advantage of the universal motor is that AC supplies may be used on motors
which have the typical characteristics of DC motors, specifically high starting
torque and very compact design if high running speeds are used. The negative
aspect is the maintenance and short life problems caused by the commutator. As
a result such motors are usually used in AC devices such as food mixers and
power tools which are used only intermittently. Continuous speed control of a
universal motor running on AC is very easily accomplished using a thyristor
circuit, while stepped speed control can be accomplished using multiple taps on
the field coil. Household blenders that advertise many speeds frequently
combine a field coil with several taps and a diode that can be inserted in series
with the motor (causing the motor to run on half-wave rectified AC).
1. The universal motor can operate in AC only.
2. Impedence is not important in designing a universal motor.
3. It is dificult for a universal motor to produce the output of over one
kilowatt when operating at normal power line frequencies.
4. The commutator can work for a short time.
5. A thyristor circuit is a solution to continuous speed control of a universal
motor running on AC

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Exercise 5
useful
into

Choose a correct word to fill in the gap
rotate
permanent

limit
due to

cost
exceed

Universal motors can ………………………. (1)at relatively high revolutions
per minute (rpm). This makes them ……………………. (2)for appliances
such as blenders, vacuum cleaners, and hair dryers where high-speed
operation is desired. Many vacuum cleaner and weed trimmer motors exceed
10,000 rpm, Dremel and other similar miniature grinders will often
………………………….(3)30,000 rpm. Motor damage may occur
……………………. (4)overspeed (rpm in excess of design specifications) if
the unit is operated with no significant load. On larger motors, sudden loss of
load is to be avoided, and the possibility of such an occurrence is
incorporated …………………… (5)the motor's protection and control
schemes. Often, a small fan blade attached to the armature acts as an artificial
load to …………………… (6)the motor speed to a safe value, as well as
provide cooling airflow to the armature and field windings.
With the very low ……………………… (7)of semiconductor rectifiers,
some applications that would have previously used a universal motor now
use a pure DC motor, sometimes with a ……………………. (8)magnet field.
Exercise 6 Complete the sentences by finding the missing letters
1. The energy acquired by the objects upon which work is done is known as
m…………….l energy.
2. In electrical engineering, an a……………….e is one of the two principal
electrical components of an electromechanical machine--a motor or generator.
3. Electrical i…………………e describes a measure of opposition to a
sinusoidal alternating current (AC).
4. A t………………….n motor is a type of electric motor used to power the
driving wheels of a vehicle such as a railroad locomotive, electrical multi-unit
train (such as a subway or light rail vehicle train), a tram, or an automobile.
5. A l…………………e is a railway vehicle that provides the motive power for
a train.

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6. A t………………e (τ) in physics, also called a moment, is a vector that
measures the tendency of a force to rotate an object about some axis.
7. A c………………….r is an electrical switch that periodically reverses the
current direction in an electric motor or electrical generator.
8. R…………………….ns per minute (abbreviated rpm, RPM, r/min, or
r·min−1) is a unit of frequency: the number of full rotations completed in one
minute around a fixed axis.

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